Limitations of Spectral Measurements

Spectral measurements of optical filters using standard spectrometers suffer from a few limitations, especially when attempting to measure very steep edges and sharp transitions.

1. UNCOLLIMATED BEAM
   Standard spectrometers generally have a sample compartment where the sample is positioned at a beam focus. The beam typically has a CHA in the 3-5º range. This results in a spectrum that is /integrated over all subsumed angles, softening the edge steepness and corners in the spectrum.
   
   
2. LIMITED SENSITIVITY
   Even with reference beam attenuation, it’s difficult to achieve OD measurements above OD=7. As the measurements approach this limit, the spectrum begins to deviate from the predicted performance in a standard spectrometer.
   
   
3. TRADE-OFFS BETWEEN SPECTRAL RESOLUTION AND SENSITIVITY

To measure high OD values, customers often open the slits of the spectrometer to increase incident light intensity. In so doing, spectral details are smoothed out over a number of wavelengths and “measurement artifacts” are observed in the steep edges. Reducing the slit width will remove the artifacts and increase spectral resolution, but result in a reduction of OD measurement sensitivity. 

FIGURE 1. IDEX Health & Science custom-designed Peregrine and KolaDeep^™ SMS systems enable measurement of the steepest edges and deepest blocking.

IDEX Health & Science has addressed these issues by designing custom spectral measurement systems. These custom systems enable our team to accurately measure, test, and verify designs with deep blocking and/or steep spectral edges.

Our workhorse is the Peregrine SMS. It was designed with a CHA <0.3º to eliminate one of the major sources of error described above. It has allowed IDEX Health & Science to consistently lead the way in the development of extremely steep edges and multi-band designs for our Semrock optical filters.

KolaDeep SMS is our newest spectral measurement system- it combines the features of the Peregrine SMS sample configuration with a proprietary optical design to deliver best-in-class sensitivity without compromising spectral resolution. It can measure OD>8 from 275–800 nm and OD>7 from 800– 1100 nm while simultaneously accurately measuring the entire edge slope and transmission region. 

• Learn More About Our Unique Spectral Measurement Capabilities
• Download Our New Optical Filters 101 Guide